1. Field of the Invention
The present invention relates to a speaker apparatus and a sound reproduction apparatus; more specifically, to a speaker apparatus and a sound reproduction apparatus suitable for use in vehicles and automobiles.
2. Description of the Prior Art
Along with the increasing popularity of digital audio media, such as compact discs, mini-discs (MD), and digital video discs (DVD), the sound reproduction apparatus must be able to reproduce the wide range signals recorded in these media.
A passive radiator type Kelton system, which is a combination of a passive radiator and a speaker unit, is a conventional way for improving the sound reproduction capability of a speaker apparatus in a low frequency range (low range).
A conventional speaker apparatus of passive radiator type Kelton system is described in the following. FIG. 22 shows an exploded perspective view of the conventional speaker apparatus.
In FIG. 22, a speaker unit 1202 drives a passive radiator unit 1201. The passive radiator unit 1201 is mounted on a front baffle 1203, which is a constituent part of a speaker box. A closed front chamber 1204 couples sound output of the speaker unit 1202 with the passive radiator unit 1201. A closed rear chamber 1205 confines sound output generated from the back of the speaker unit 1202. The speaker unit 1202 is mounted on a sub baffle plate 1206, which separates a space into the closed front chamber 1204 and the closed rear chamber 1205. A back plate 1207 and the front baffle 1203 form a speaker box.
Operation of the above-configured conventional passive radiator type Kelton system speaker apparatus is described below.
As shown in FIG. 22, the sound reproducing output (hereinafter referred to as sound output) irradiated from the front of the speaker unit 1202 mounted on the sub baffle plate 1206, which has been disposed for separating the closed front chamber 1204 from the closed rear chamber 1205, drives the passive radiator unit 1201 mounted on the front baffle 1203. The sound output is conveyed by the air within the closed front chamber 1204 formed by the speaker unit 1202, the front baffle 1203 and the sub baffle plate 1206. The passive radiator unit 1201 reproduces the sound. The sound output irradiated from the back of the speaker unit 1202 is confined in the closed rear chamber 1205 formed by the speaker unit 1202, the front baffle 1203, the sub baffle plate 1206 and the back plate 1207, in order not to cause interference with the sound output of the passive radiator unit 1201.
FIG. 23 compares the low range reproduction characteristics, for demonstrating the superiority of a conventional passive radiator type Kelton system. In FIG. 23, curve 1301 represents the frequency characteristics of output sound pressure in a closed chamber speaker system. The closed chamber confines the sound output generated from the back of the speaker unit within the speaker box for the purpose of preventing interference with the sound output generated from the front of the speaker unit. However, in a case where the volume of the speaker box is not sufficiently large, the compliance to the speaker unit deteriorates and the capability of low range reproduction is reduced as seen in FIG. 23. Curve 1302 represents the frequency characteristics of output sound pressure in a phase reversion system using the same speaker unit and the same speaker box. In the phase reversion system, sound output generated from the back of a speaker unit is put to a resonance at a certain specific frequency (non-resonating frequency) through a duct to be mixed with sound output of the speaker unit in the front. Since the sound output coming through the duct is in the same phase as that of the front sound output of the speaker unit in a frequency region above the non-resonating frequency, the radiation efficiency is increased as a result of the mutual action. Therefore, the limitation in low range reproduction is improved. In a very-low frequency range, however, the phase of the sound output coming through the duct is reversed relative to that of the sound reproducing output from the front of the speaker unit; which causes a cancellation to each other. Therefore, the sound output sharply attenuates for approximately −20 dB/oct in the very-low frequency range. So, reproduction of the heavy low sound remains insufficient.
Curve 1303 represents output sound pressure frequency characteristics in a conventional passive radiator type Kelton system using the same type speaker unit and speaker box. The passive radiator type Kelton system can extend the limitation of low range reproduction, by having each of the closed chambers formed by the passive radiator unit, the speaker unit and the speaker box at a certain specific frequency, like in the phase reversion system. Since the sound reproduction output of the passive radiator unit and that of the speaker unit are not mixed together even in the very low frequency region, the attenuation curve in the very low frequency band region remains as moderate as that of a closed chamber system, or approximately −12 dB/oct. Thus, the heavy low sound is reproduced sufficiently. Furthermore, since a diaphragm of the passive radiator does not vibrate in a frequency region above a certain specific frequency even when a speaker unit vibrates, it provides a superior band pass characteristic as a low range speaker apparatus.
As described above, the conventional passive radiator type Kelton system offers both of the advantages; viz. it is capable of reproducing heavy low sound, which is the advantage of a closed chamber system, and it can extend the limitation of low range reproduction, which is the advantage of a phase reversion system. Thus, the low range reproduction capability has been improved.
However, the conventional passive radiator type Kelton system speaker apparatus has a unique and complicated structure of separating the inside of a speaker box into a closed front chamber and a closed rear chamber using a sub baffle plate. Therefore, it is difficult for the speaker box to take a slim contour. Furthermore, since a speaker unit is totally housed within a speaker apparatus, it has a poor heat dissipation characteristic, which results in a poor withstanding capability against high inputs.
Furthermore, since a speaker unit, or the source of vibration, has been fixed on a sub baffle plate, unwanted vibration of the sub baffle plate causes abnormal sound generation. Thus, it is difficult for the conventional structure to implement a speaker box in small overall dimensions.